The present invention relates generally to joints of drill pipe for rotary earth drilling having tool joint members on each end with threaded connections for connecting the joints with other joints of drill pipe and, specifically, to such tool joint members having improved frictional properties at elevated temperatures, and a method of connecting the same.
A drill pipe string of the type used in rotary earth drilling is comprised of joints of drill pipe which have box and pin members secured to the opposite ends thereof by suitable means, such as welding. The "box" tool joint member is internally threaded and adapted to receive a mating externally threaded "pin" tool joint member. Mating joints of drill pipe are interconnected to form the pipe string used in the rotary earth drilling process to suspend, support and rotate a drill bit and produce a borehole. It is necessary that the joints of drill pipe be made up tightly enough to prevent leakage, wobble, and unscrewing. Power tongs and power driven cable winches are often used to provide the torque to make up the tool joint connections. The frictional properties of the threaded connections of the tool joint members are important since these frictional properties dictate the amount of torque that can be transmitted by the tool joint. The higher the friction coefficient, the greater the torque transmitting capability and the less the chance of additional makeup occuring down the hole. Insufficient frictional engagement can allow excessive makeup which can burst the box or stretch, crack or completely break the pin.
In order to maintain a high coefficient of friction between the threaded connections of the box and pin tool joint members, zinc base thread compounds are widely used. The zinc base thread compound or "dope" allows the tool joints to be broken out easily and also minimizes excessive make up during drilling. A problem exists, however, with the use of zinc base thread compounds in high temperature drilling environments such as in geothermal wells. The zinc base thread compound tends to lose its lubrication properties at higher temperatures and it is then difficult to break the tool joint connection when tripping pipe out of the well bore. Because of this problem, copper base thread compounds are traditionally used in geothermal wells and other high temperature drilling environments. The copper base thread compound maintains its lubrication properties at higher temperatures allowing the tool joints to be broken out more easily. However, because the coefficient of friction of the copper base thread compound is lower, torsional failures are more likely since additional makeup occurs between the pin and box resulting in stretched pins and other problems.
It has now been discovered that by nickel plating the tool joint threaded connections, zinc base thread compounds can be utilized without producing the undesirable effects generally associated with zinc base compounds at higher temperatures.